Cracking Control

NMR Application – Fluid Catalytic Cracking Control and Optimization

The fluid catalytic cracking unit (FCCU) is one of the most important units in the refinery. Few FCCUs have real-time process optimization implemented, since feeds typically have been measurable only in the laboratory. These measurements take many hours, with reports available only once or twice a day. Even the measurement of PIONA (paraffins, isoparaffins, olefins, naphthenes, and aromatics) and the distillation properties of the rundowns are difficult to achieve on-line.
The Donahue Process Systems NMR now provides a means of obtaining these measurements near real-time, thus enabling significant economic benefit to the refiner through APC and process optimization.
FCCU controls and optimization include feed preparation, the reactor/regenerator, the main fractionator, the wet gas compressor, and the downstream gas plant. Typical operating objectives are:

Maximizing unit capacity

Maintaining product quality while maximizing yields of most valuable products

Optimizing energy utilization

Controlling conversion

Improving safety and reliability via operational stability

Total economic benefit can approach $0.30/bbl feed.

Product 1: NMR Prediction of FCC Feed Characteristics

In this application the process NMR unit provides control parameters based on the needs of the plant. Different plants have different chemical and parameters of the FCC Feedstream to control or optimize the FCC unit.
Typical parameters requested for the NMR are:

Density

Sulphur **

Total Nitrogen **

Basic Nitrogen **

Metals ++

Micro Conradson Residue

Aniline Point

Refractive Index

Total Aromatics

Aromatic Breakdown (Mono, di, tri, tetra+ rings)

Distillation (D1160)

Viscosity

Hydrogen Content

Asphaltenes

** Secondary correlation
++Cannot be determined by NMR directly-Relaxation studies may allow quantification of Total Metals (No discernment between metal types).
All the above parameters can be modeled (except metals). Sulphur and nitrogen numbers will be secondary correlations that can be used only for gross estimation. These are poorer correlations because the NMR does not observe N or S directly, instead correlation is found in H chemistry that is elated to these heteroatoms. The remaining parameters can be modeled to an error of prediction somewhere between ASTM repeatability and reproducibility for the ASTM tests depending on the validation procedure that is followed.
Process NMR Associates is also developing large databases that correlate 1H NMR spectra with 13C NMR parameters that are more useful and technically accurate than the above tests many of which simply correlate to the 13C NMR parameters of carbon aromaticity (FA), carbon paraffinicity (FP) and carbon naphthenicity (FN). These parameters would be the preferred parameters for online control and optimization.
A separate description of these 13C NMR parameters is given here.

Product 2: NMR Prediction of FCC Product Characteristics

Characteristics of these streams (FCC gasoline and Cycle Oils) are closely related to other applications such as gasoline blending and gas oil blending. Relevant information can be found in the description of these applications.
For further process details: The FCC Network